Methylamine compounds are usually produced from methanol and ammonia using a solid acid catalyst such as silica-alumina at a temperature of around 400° C. As is well known, use of the silica-alumina catalyst leads to predominant production of trimethylamine according to thermodynamic equilibrium although trimethylamine is least demanded among the three types of methylamines, namely, mono-, di- and tri-methylamines. However, dimethylamine accounts for most of the demand for methylamines, and least demanded trimethylamine is recycled for a reaction system after being subjected to separation from a reaction product by distillation, resulting in a very large amount of consumption energy. For this reason, recently, methods for selectively producing dimethylamine that overcome the thermodynamic equilibrium composition have been developed.
Examples of such methods include those using zeolites (crystalline aluminosilicate molecular sieves) such as Zeolite A (for example, see Patent Document 1), FU-1 (for example, see Patent Document 2), ZSM-5 (for example, see Patent Document 3), ferrierite and erionite (for example, see Patent Document 4), ZK-5, Rho, chabazite and erionite (for example, see Patent Document 5) and mordenite (for example, see Patent Documents 6, 7, 8 and 9). In these methods, zeolites having a small micropore channel size are further subjected to ion exchange, dealumination treatment, addition of a specific element, silylation treatment or the like in order to control the micropore channel size or modify acid sites on external surfaces thereof, thereby trying to improve dimethylamine selectivity and catalytic activity.
Further, for example, a method for producing a methylamine compound using a crystalline silicoaluminophosphate salt molecular sieve that overcomes the thermodynamic equilibrium composition (for example, see Patent Document 10) is also publicly known. The present inventors made researches on techniques of selectively producing dimethylamine and found that SAPOs modified with silica, SAPOs modified with various oxides and SAPOs in which an amorphous oxide layer having an appropriate thickness is formed on surfaces of crystal particles exhibit activity and dimethylamine selectivity higher than those of catalysts of prior art, and have already filed patent applications related thereto (for example, see Patent Documents 11, 12, 13 and 14). In addition, Patent Document 11 describes the production of dimethylamine by means of disproportionation of monomethylamine.
By such improvement of catalysts, the cost for the production of methylamines has been significantly improved compared to those of processes using conventional catalysts. However, from a practical viewpoint, long-term stability of catalytic performance is desired to be further improved, and temporal stability of dimethylamine selectivity and long-term maintenance of catalytic activity are desired.
Crystalline aluminosilicate molecular sieves and crystalline silicoaluminophosphate salt molecular sieves are sometimes modified by means of contact with water vapor for the purpose of the improvement of catalytic activity and selectivity when used as catalysts for the production of chemical products. For example, ultrastabilized Y-type zeolite (USY), which is used for fluid catalytic cracking (FCC), is obtained by being contacted with water vapor at 600 to 800° C. (for example, see Patent Document 15 and Non-patent Document 1). Further, Barger et al have reported that, when a crystalline silicoaluminophosphate salt molecular sieve is treated under water vapor atmosphere at 700 to 900° C., C2-C3 olefin selectivity and catalyst life are improved in a methanol conversion reaction (see Patent Document 16).
Modification of catalysts for methylamine synthesis by means of contact with water vapor is also publicly known. For example, Patent Document 9 describes that dimethylamine selectivity is improved by contacting a crystalline aluminosilicate molecular sieve such as mordenite with water vapor at 250 to 700° C. However, even though selectivity is improved, the effect thereof is not sufficient, and catalytic activity is sacrificed for the contact with water vapor.
Modification using water vapor is sometimes carried out for the purpose of the improvement of the strength of a molded body or removal of impurities in a catalyst. For example, Patent Document 17 discloses a method for improving the strength of a molded body comprising a crystalline aluminosilicate molecular sieve by means of the treatment under a flow of water vapor-containing gas at 100 to 600° C. Further, Patent Document 18 discloses a method in which a crystalline silicoaluminophosphate salt molecular sieve molded using a halogen-containing binder is contacted with water vapor at 400 to 1000° C. to remove halogen in the catalyst. The documents disclose that these catalysts can be used for methylamine synthesis reaction, but do not describe any effect on catalytic activity and selectivity.
As described above, there are many reports regarding methods for modifying crystalline aluminosilicate molecular sieves and crystalline silicoaluminophosphate salt molecular sieves by means of the contact with water vapor, but no convenient and effective method has been found for improving catalytic activity and selectivity in the production of methylamines and maintaining activity and selectivity for a long period of time.